New research suggests that the combination of limited sunlight exposure (vitamin D deficiency), coupled with a history of having a common illness known as mononucleosis (Epstein-Barr virus infection) may explain much of the risk for development of multiple sclerosis in genetically susceptible individuals. (And raising the question of whether some such combination might trigger ME/CFS - once termed 'atypical MS, and often preceded by mono.)

"MS is more common at higher latitudes, farther away from the equator," says Prof. George C. Ebers, MD, an MS expert at the University of Oxford and co-author of the study.

"Since the disease has been linked to environmental factors such as low levels of sun exposure and a history of infectious mononucleosis, we wanted to see whether the two together would help explain the variance in the disease across the United Kingdom," he explains.

• Infectious mononucleosis is a disease caused by the Epstein-Barr virus, which is a Herpes virus that is extremely common but causes no symptoms in most people.

• However, when a person contracts the virus as a teenager or adult, it often leads to infectious mononucleosis.

• The body makes vitamin D when exposed to ultraviolet B (UVB) light (and in absence of sufficient sun exposure the vitamin can be supplied by foods such as fortified milk and supplements).

For the study, researchers looked at all hospital admissions to National Health Service hospitals in England over seven years. Specifically, they identified:

• 56,681 cases of multiple sclerosis

• And 14,621 cases of infectious mononucleosis.

The researchers also looked at NASA data on ultraviolet intensity in England.

The study found that adding the effects of sunlight exposure and mononucleosis together explained 72% of the variance in the occurrence of MS across the UK. Sunlight exposure alone accounted for 61% of the variance.

"It's possible that vitamin D deficiency may lead to an abnormal response to the Epstein-Barr virus," Dr. Ebers suggests.

He notes that low sunlight exposure in the spring was most strongly associated with MS risk. "Lower levels of UVB in the spring season correspond with peak risk of MS by birth month,” he says. “More research should be done on whether increasing UVB exposure or using vitamin D supplements and possible treatments or vaccines for the Epstein-Barr virus could lead to fewer cases of MS."

The study was supported by the Multiple Sclerosis Society of Great Britain and Northern Ireland, the Medical Research Council and the Wellcome Trust.